It is an Android 4.2 feature so I'm sure it's also on the Nexus 7 and Nexus 4. But those devices are smaller and less likely to be shared so they are not playing multi-user up in those marketing materials I suspect.
"Available only on tablets.", see http://www.android.com/whatsnew/
I think there's a certain sense that a phone is a much more "personal" device than a tablet. While it would probably be possible to add multi-user support, it would be more difficult, and be less useful, than on a tablet.